1. Technical Field
The field of the invention is the mapping of high layer packets into lower layer frames in a communication system, which can be either a wireless or fixed line network. In order to adapt the packets delivered by the upper layer to the capabilities of the physical network (e.g., maximum frame size), it is sometimes necessary to segment or fragment them into several blocks that would be transmitted by separate frames. In the same way, it might be also necessary to concatenate several packets into one frame in order to increase transmission efficiency. The invention proposes a new and efficient way of indicating to the receiving unit how segmentation and concatenation has been done at the transmitter side.
2. Description of the Related Art
The necessity of adapting higher layer packets to the characteristics of a physical network is a classical issue for all type of communication systems, such as wireless networks (GSM, UMTS, WiLAN, WiMAX, etc.) or fixed networks (IP, Frame relay, PPP, ATM, etc.).
General Overview of the OSI Layer
In this section, a brief introduction is given to the OSI model (see FIG. 1) that will be used to illustrate the explanations below.
The Open Systems Interconnection Reference Model (OSI Model or OSI Reference Model for short) is a layered abstract description for communications and computer network protocol design. The OSI model divides the functions of a system into a series of layers. Each layer has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. A system that implements protocol behavior consisting of a series of these layers is known as a ‘protocol stack’ or ‘stack’. Its main feature is in the junction between layers which dictates the specifications on how one layer interacts with another. This means that, in principle, a layer written by one manufacturer can operate with a layer from another. For our purpose, only the three first layers will be described.
The main purpose of the physical layer, or layer 1, is the transfer of information (bit) over a specific physical medium (e.g., coaxial cables, twisted pairs, optical fibers or the air). It converts or modulates data into signals that are transmitted over a communication channel.
The purpose of the data link layer, or layer 2, is to shape the information flow in a way compatible with the specific physical layer by breaking up the input data into data frames (Segmentation And Re-assembly or SAR functions). Furthermore it may detect and correct potential transmission errors by requesting a retransmission of a lost frame. It provides an addressing mechanism and may offer flow control algorithms in order to align the data rate with the receiver capacity. Finally, when a shared medium is concurrently used by multiple transmitter and receivers, it offers mechanisms to regulate and control access to the physical medium. As the span of functions of the data link layer is large, the data link layer is often subdivided in two sublayers (e.g., RLC and MAC sublayers in UMTS). Typical examples of layer 2 protocols are PPP/HDLC, ATM, frame relay for fixed line networks and RLC, LLC or MAC for wireless systems.
The network layer, or layer 3, provides the functional and procedural means for transferring variable length packets from a source to a destination via one or more networks while maintaining the quality of service requested by the transport layer. The main purposes of the network layer are to perform network routing, network fragmentation and congestion control functions. The main examples of network layer protocols are the IP Internet Protocol or X.25.
More information on OSI layer model can be found in “Computer Networks”, (Andrew S. Tanenbaum, fourth edition, Prentice Hall International Edition, pages 37-41, section 1.4).